1. Field of the Invention
The present invention relates to a plasma processing system for Transmission Electron Microscopy, or TEM, specimens and specimen holders. More specifically, the system utilizes a low energy, high frequency or HF, plasma to remove amorphous damage resulting from various specimen preparation techniques from TEM specimens and to remove contamination from TEM specimens and specimen holders.
2. Description of the Prior Art
Transmission Electron Microscopy, or TEM, technology enables materials to be analyzed at near atomic resolution by providing high magnification, high resolution imaging and analysis capabilities. TEM enables scientists to gather information relating to a material's physical properties such as its microstructure, crystalline orientation and elemental composition. This information has become increasingly important as the need for advanced materials for use in areas such as microelectronics and optoelectronics, biomedical technology, aerospace, transportation systems and alternative energy sources, among others, increases.
TEM is accomplished by examining material specimens under a transmission electron microscope. In a transmission electron microscope, a series of electro-magnetic lenses direct and focus an accelerated beam of electrons, emitted from an electron gun contained within the microscope, at the surface of a specimen. Electrons transmitted through the specimen yield an image of the specimen's structure which provides information regarding its properties. In addition, elemental and chemical information is provided by both the transmitted electrons and the x-rays that are emitted from the specimen's surface as a result of electron interaction with the specimen. Thus, because it is necessary for the electron beam to transmit through the specimen, a key component of successful material analysis by TEM techniques is the condition and preparation of the specimen itself.
Before a specimen can be analyzed using TEM, it must be prepared using various techniques to achieve the necessary electron transparency. This electron transparency is accomplished by thinning a defined area of the specimen. For equal resolution, the required thickness of the specimen is dependent on the accelerating voltage of the transmission electron microscope. For a 120 kV microscope, the specimen thickness must be on the order of 100 to 2000 angstroms. A 1,000 kV microscope can tolerate a specimen thickness of up to 5,000 angstroms.
Specimens are prepared through several well known methods, including, but not limited to, electrolytic thinning, mechanical grinding, ultramicrotomy, crushing, and ion milling. Often times, multiple methods are utilized to prepare a single specimen. For most types of specimens, either electrolytic thinning or ion milling is used as the final form of specimen preparation. In both cases, amorphous damage ranging in thickness from 1-10 nanometers may result, particularly in the case of ion milling. In this case, the energy of the ion beam transforms the crystalline structure of the material to an amorphous state. This amorphous damage adversely effects the quality of the TEM analysis because it alters the natural characteristics of the material.
Once prepared, the specimen is placed into the microscope's specimen holder, which is an individual component separate from and external to the microscope itself. The specimen holder is then inserted into a goniometer located within the column of the microscope. The goniometer provides X-Y-Z and tilt manipulation of the specimen and places the specimen between the lenses of the microscope.
In addition to amorphous damage resulting from the preparation process, another problem which adversely affects the quality of the TEM analysis is hydrocarbon contamination of the specimen and specimen holder. This contamination can occur as a result of poor operator handling techniques during the preparation process, such as touching the specimen and/or specimen holder with an ungloved hand. Other contamination may result from subjecting the specimen to an ion milling system that utilizes an oil diffusion pump whereby backstreaming of oil will lead to contamination, the use of hydrocarbon based solvents and adhesives in the preparation process, storage of the specimen holder in ambient conditions, and repeated exposure to the microscope's vacuum system which may contain oil vapor which has migrated up the electron optics column from an oil diffusion pump. Although contamination of specimens mainly consists of hydrocarbon compounds, other types of contamination, such as oxides or particulates, can be present.
Furthermore, in recent years, transmission electron microscope manufacturers have increased the current density of the electron beam through advancements in electron gun technology. Beam current density increases with the brightness of the gun as shown in Table I.
TABLE I ______________________________________ Type of Gun Technology: Beam Current (for a 10 angstrom probe size): Tungsten Filament 1 to 1.5 picoamps LaB.sub.6 10 to 15 picoamps Field Emission Gun (FEG) 0.5 to 0.6 nanoamps ______________________________________
With increased brightness and electron beam current density comes increased resolution and enhanced analytical capabilities. However, increasing brightness and beam current also increases the localized energy of the electron beam, which, in turn, increases the attraction of hydrocarbon compounds to the beam. As a result, as the brightness and electron beam current density increase, so does the tendency for the hydrocarbon contamination to migrate to the impingement point of the electron beam on the specimen. As this migration occurs, carbon formations obstruct that particular area of the specimen from both image observation and the possibility of acquisition of analytical information. Thus, with increased brightness and electron beam current density comes an increased need for a contamination-free specimen.
Richard S. Thomas and John R. Hollohan, in an article entitled "Use of Chemically-Reactive Gas Plasmas in Preparing Specimens for Scanning Electron Probe Microanalysis", Proceeding of the 7th Annual Scanning Electron Microscope Symposium, Scanning Electron Microscopy, pp. 84-89, 1974, disclosed the use of an oxygen plasma for etching and ashing scanning electron microscopy, or SEM, specimens and for cleaning carbonaceous material from SEM specimens and instrument parts. The use of a pure oxygen plasma, however, was not demonstrated to have reduced the amorphous damage of all specimens.
H. W. Zandbergen, et al., "The Use of Plasma-Cleaning for the Preparation of Clean Electron-Transparent Thin Foils," Proceeding of the XIII International Congress for Electron Microscopy, Volume 1, pp. 1003-1004, 1994, discloses the use of plasma cleaning, specifically low energy argon plasma, for the reduction of the amorphous layer and carbon contamination in TEM specimens, the latter being shown to be reduced by a factor of 100.
Furthermore, it is well known in the art to use a plasma generated from ambient air, which essentially comprises an approximate mixture of 80% nitrogen and 20% oxygen, for processing of TEM specimens.
What is not disclosed, however, and thus what is lacking in the prior art is an integrated processing method and apparatus therefor which may be used prior to a specimen's insertion into a transmission electron microscope that removes both amorphous damage and contamination from TEM specimens and specimen holders.